The present invention relates to oil pump apparatus for supplying oil to a transmission of a vehicle.
A patent document 1 (JP2001-165286 A) shows a conventional pump system including a main oil pump driven by an engine and a sub oil pump driven by a differential gear. In order to hold the sub oil pump inoperative in the normal vehicle travel operation, the suction opening of the sub oil pump is placed at the position higher than the suction opening of the main oil pump. When the oil level becomes lower by the operation of the main oil pump, at least part of the suction opening of the sub oil pump is exposed above the oil level, and hence the sub oil pump becomes unable to function. Thus, the pump system holds the sub oil pump inoperative in the normal vehicle travel operation and avoids power loss by the sub oil pump.